Image size

hello all! Can you suggest me which format is the image less heavy (in size).

I must create an application with 700 / 800 photos…what format to use? I need a medium quality

Why not just go to texture inspector, set Texture type to Advanced, and change the format to one you like, if the quality/size is sufficient for you, use it.

How are you planning to use them? As GUI? As textures in-game?

It doesn’t matter what format you use. Unity does not leave them in their format anyway but an own internal interpretation basing on the compression - color format you specify