Add back max size for images support - for remote images

[OctoDev]

On older versions such as 3.4, when you added images to your signature it would check the image for size (Height, width and general file size).. However in 4.* you can upload any size images to signature, using a remote url.

I think this is really annoying, since i rather not have 5mb signatures or even wallpapers as signatures.

At the moment, maximum size of signature is only uploaded files - which makes no sense since anyone can simply put a url link and bypass that.

I did notice that it checks for height and width but not SIZE.