@jildyso: The ability to highlight a single image in a photo does depend on the photo itself, as it is not always possible. If you press and hold on a photo, does a pop-up appear that says 'Couldn't find anything to clip'? If so, as a workaround, tap on the pencil symbol at the bottom of the screen > Tap the 4 circles in a square shape > Lasso > Select manually > Shape > Select a shape > Drag the white border to adjust the size of the shape > Done > Tap the 3 dots in the top right > Save as copy.
Now, select the newly made image > Pencil symbol > Smiley face symbol with a pencil inset > Stickers > Gallery symbol (to the left) > + > Follow the on-screen instructions.
Alternatively, try zooming in on the object you want to isolate, then press and hold it again. If successful, a 'Save as sticker' option should appear, which will isolate the image. This won't remove the background, but will allow you to use the custom sticker when editing other images.
