Some limitations on image filesizes in signatures have been increased. Signature images may now be up to 100 KiB in size and you may use up to 8 images.
The previous limitations were no more than 5 images, with each image being limited to 20 KiB maximum. That came up to a total limit of 100 KiB. The changes allow more flexibility: you can use the full 100 KiB for one image or spread it out as you like across 8 images.
Thanks to xellos667 for the initial suggestion of increasing the individual image size.