Changing the source image of a UI Image element in the editor - simple, surely?

I’ve changed my mind about how I want a heads up display image to look, but I cannot for the life of me figure out how to correctly import the new image (I want to retain the old one, in case my idea doesn’t work).

I have put the new image in my Assets folder, and it’s showing as present in the Unity project panel, but when I try to drag it to the Source Image slot on my UI Image prefab, it won’t have it (It makes that black circle-with-a-diagonal-cross symbol). I’ve also tried clicking the ‘cog’ to the right of the Source Image slot, but my new image is not featured in those available. I’ve also tried Assets > Import New Asset, but the imported image gets the same ‘black circle symbol’ treatment when I try to add it to the Source Image slot.

Sorry to have to ask what SHOULD be obvious, but what am I doing wrong?

EDIT: I should have said all of the images I’ve imported are PNGs. I’d like to know how to import them in a way that’s useful in Unity. It seems like the earlier image has been imported as ‘NPOT ARBG 32 bit’ ; How do I import new images in that format?

Select your image in the Project window and then change the image type from " Texture" to " Sprite 2d and UI"

Hey,

The thing you are importing is not a image format that Unity knows. It must be SD, TIFF, JPG, TGA, PNG, GIF, BMP, IFF or PICT.

Cheers,