<b>A maximum width of 550 pixels.
A maximum height of 250 pixels.
These include any text in the signature outside images.
<font color=yellow>Total file size of all images combined must not exceed 50 KiB (51,200 bytes).</font>
Any signatures not following these or any signatures with questionable or problematic content will be removed/edited by staff, and you may have your signature editing privileges temporarily disabled if this happens several times.</b>

I'm gonna take a wild guess and say you broke the "Total file size of all images combined must not exceed 50 KiB (51,200 bytes)." rule.