This was an enjoyable, if somewhat easy, puzzle from Tuesday regular, Armonie, to whom thanks. Definitions are underlined in the clues. [[The pictures at the bottom have unidentified links to the puzzle. Please enclose any comments on them in double brackets. Thank you.]]
Across
1 Union supporter trips in British Museum – help needed (10)
BRIDESMAID : RIDES(trips on horseback or in vehicles, say) contained in(in) BM(abbrev. for “British Museum”) plus(… needed) AID(help;assistance).
Defn: Cryptic – A supporting role in a wedding;a matrimonial union.
7 Fish for fingerprints (4)
DABS : Double defn: 1st: Any of various flatfish; and 2nd: Slang for fingerprints.
9 Primate holds beginning of service in church (4)
APSE : APE(any of various primates, with very short or no tails) containing(holds) the first letter of(beginning of) “service “.
Answer: A recess in a church (building).
10 The samurai becomes incompetent (10)
AMATEURISH : Anagram of(becomes) THE SAMURAI.
11 John worked for Frank (6)
CANDID : CAN(slang for the toilet, as is “john”) + DID(worked, as in “he did the morning shift”).
Defn: With a lower case “f”.
12 Trying to impress improperly? Pardon me! (4-4)
NAME-DROP : Anagram of(improperly) PARDON ME.
13 Witchcraft leads to shortage in pub by Tuesday (5,3)
BLACK ART : LACK(a shortage of) contained in(in) BAR(a pub;a watering hole) plus(by) T(abbrev. for “Tuesday “).
15 Very last trade (4)
VEND : V(abbrev. for “Very“) + END(the last bit).
Answer: To sell or trade, as what a vendor does for a living;a trade.
17 Caught in flagrante delicto in India (4)
AGRA : Hidden in(Caught in) “flagrante delicto “. Smooth surface.
Defn: A city in India.
19 Asian has fence going in two directions (8)
NEPALESE : PALE(a fence;an enclosing barrier) contained in(going in) NE,SE(abbrevs. for the 2 directions, northeast and southeast, respectively).
22 Underwear displayed in bed (8)
BLOOMERS : Double defn: 2nd: What may be displayed in a flower bed – if you are or have a good gardener.
23 Reprimand sailor (6)
RATING : Double defn: 1st: A severe scolding; and 2nd: An ordinary seaman in the navy.
25 Ornament broken? Cleaner hid! (10)
CHANDELIER : Anagram of(broken) CLEANER HID.
26 Limit for writer drinking port (4)
POLE : POE(Edgar Allan, American writer) containing(drinking) L(abbrev. for “port”;left side on a vessel when facing the bow).
Answer: Either of the two ends;limits of the axis of a sphere, body or organ.
27 Oscar gets some French poetry (4)
ODES : O(the letter represented by “Oscar” in the phonetic alphabet) plus(gets) DES(French for the article “some”).
28 Considered giving shop censure (10)
DELIBERATE : DELI(sort for “delicatessen;a shop selling various already cooked or prepared food, especially imported ones) + BERATE(to censure;to reprimand).
Defn: As an adjective, as in “one’s considered opinion”.
Down
2 Hear pal has become a painter (7)
RAPHAEL : Anagram of(… has become) HEAR PAL.
Self and mistress’s portraits by the artist.
3 King comes in late for quail (5)
DREAD : R(abbrev. for the Latin “Rex”;king) contained in(comes in) DEAD(the euphemism for which is “late”). An interesting surface – a king arriving late for his dinner of quail.
Defn: To shrink with fear.
4 Suffer gentle slap but refuse to change (5,3)
STAND PAT : STAND(to suffer;to bear;to tolerate) + PAT(a gentle slap). Another nice surface – it might take more than a gentle slap to achieve the desired change.
5 Once more measure refuse reluctantly (7,4,4)
AGAINST ONE’S WILL : AGAIN(once more) + STONE(a unit measure of weight in the Imperial System) + SWILL(kitchen refuse;garbage in general).
6 Duke has loads of fantasies (6)
DREAMS : D(abbrev. for “duke”) plus(has) REAMS(a large quantity;loads of).
7 Stunt man died? That’s a ruddy disaster! (9)
DAREDEVIL : D(abbrev. for “died”) + A + RED(ruddy;having a red or reddish colour) + EVIL(misfortune;disaster, as in “to wish someone evil”).
8 It is played to keep fool in favour (7)
BASSOON : ASS(a fool;a stupid person) contained in(keep … in) BOON(a sought favour;a request for something to one’s benefit).
Defn: A musical instrument.
14 Compositions arranged? No chance, son! (9)
CHACONNES : Anagram of(arranged) NO CHANCE + S(abbrev. for “son”).
Defn: Musical compositions.
16 Small knife I bought initially for cut of meat (5,3)
SPARE RIB : S(abbrev. for “small”) PARER(a knife for peeling;paring fruit or vegetables) + I + first letter of(initially) “bought “.
18 Noble fellow experienced festivity in the first place (7)
GALAHAD : HAD(experienced as in “he had a dream”) placed below(… in the first place, in a down clue) GALA(a festive occasion).
Answer: The knight;noble fellow from King Arthur’s court.
20 Underwear makes one tense (7)
SINGLET : SINGLE(one in number) + T(abbrev. for “tense” in grammar).
21 Celebrity went ahead without the facts (6)
LEGEND : LED(went ahead, say, in a race or sporting match) containing(without …) GEN(informal term for information;the facts).
24 Narrow strip on bit of road (5)
TAPER : TAPE(a long narrow strip of material, eg. cloth, paper or metal) placed above(on, in a down clue) the first letter of(bit of) “road “.
Defn: To go from wide to narrow.
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For answer to pic 3 please click here.

Thanks scchua and Armonie. There were some nice long anagrams. I also liked AGAINST ONE’S WILL.
[[Pic 1 is The DREAM by Henri Rousseau, Pic 6 is Dali’s Sleep – link to DREAMS?]]
Thanks Armonie and scchua – as you say, fairly easy but enjoyable. I liked the surface and well-disguised (at least to me!) anagram in 12a.
[[Pic 2 is Sir Edmund Hillary of Everest/Sherpa Tenzing Norgay (Nepalese) fame. Pic 3 is a Picasso self-portrait which is linked to Raphael – I’ll let you fill in the details as I required the help of Google to find them! Pic 4 is Allen Funt (Candid Camera). Pics 5 and 7 are both linked to Kid Galahad films – 5 shows Bette Davis and Edward G Robinson while 7 is Elvis Presley with Joan Blackman]].
[[Muffyword and Steve, you’re both right, except for pic 3 where Picasso has a more specific link to the puzzle. I’ll wait until the end of the day to paste a link to the answer.]]
“Stand pat” comes from poker. When you refuse the offer to change cards you are said to “stand pat”.
Fun crossword so thanks to Armonie.