Hi!
So, method one would be creating a unique locked image for the images. Just drop the original locked image in an image editor, edit it how you see fit, save it under a different name and then for the gallery button you can add
add g.make_button('cg_1','cg_1_thumb', locked = "gui/gallery/alternate_bg.png")(the "locked = " part is the addition)
The second method is adding the text with code. Simply switch the gallery button to
button:
xysize(447,259) ## defining the size of the gallery button
add g.make_button('cg_2','cg_2_thumb')
label "This is a tip" align(0.5, 1.0) yoffset -35 text_color u"#ffffff" at slot_hover
action NullAction() ###leave it nullaction, it's here so the hovering transform is registered
It makes it so when the button is hovered, a text tip appears. You can style it however you want to (to make it less tedious, look into style prefixes in the documentation🙏) If you want the text to be always there, not just when hovered, just remove the "at slot_hover" and "action NullAction()" lines.