I take it that you mean the profile picture. If so, check to see that your proposed picture does not exceed any of the limits, which are 100 x 100 pixels and 100.0 KB (whichever is the smaller). In other words, you can have an image that is smaller than 100 x 100 pixels, providing that it is equal or less than 100.0 KB in size. Conversely, you can have an image that is 100.0 KB in size, providing that neither dimension exceeds 100 pixels.
Note that Avatars are subject to the same restrictions, except that the file size is 195.3 KB as opposed to 100.0 KB (the size limit for Profile images).