Well, because I cant just eyeball the size of the picture, I'm going to give you a list of things you can check against:
Images in signatures can have an accumulated limit of 500 pixels in width and 300 pixels in height (500 × 300).
The total amount of images you have in your signature cannot exceed that limit. The use of CSS in signatures cannot be used to bypass the latter limit either. A staff member has the right to disable your signature if they feel you are using CSS to bypass the 500 × 300 limit.
Text size can be no larger than 5.
Your signature cannot contain text in a higher size than 5 - or in CSS, 18pt (24px).
There are filesize limitations for images.
Your signature images may not exceed 1 megabyte in file size. Large file sizes for images are most commonly seen in large, moving .gif images. To check the size of your images on Windows, save them to a folder, right click, and under the properties tab you will find how large your image in size.
Spoiler tags are not permitted in signatures.
YouTube videos may not be embedded in signatures.
Embedded videos may not be added to your signature. A link, however, to a video you would like to share is permitted so long as it is within the text limits.
This list came directly from the Signature Rules & Review thread. If you are still unsure whether or not your Signature breaks a rule, post in that thread and ask for help.