What's the most lightweight format for textures? I want to know what's the best image format to use on textures that consumes the least ammount of VRAM during render.

I'm trying to render a scene in Blender 2.8 EEVEE but because of the textures, baking the IL takes a VERY long time and render times are around 10min mark.

I assume it's the textures that are causing this because the scene is only 1.5 million tris. But the vram consumption is 7GB but I only have a 4GB GPU (GTX970).

So I want to know which image file type will consume the least ammount of VRAM doing rendering.
The GPU does not work in terms of jpeg, png, tiff, tga, etc. During rendering every image that is used as a texture has to be stored in vram using a format that is determined by the engine, so in this case Eevee. This can range from uncompressed to various hlsl/glsl/cuda friendly compressed variants. You cannot really influence that format unless Eevee exposes its internal texture format as a config option.

The source image format you use for your textures will be converted to that internal format once, not on every frame, unless you change your source texture every frame via script or something.