I put the gif using the following simple html:
<img src="resources/si64.gif" />
into one of my test ePub (unfortunately I can't post it).
I would suggest that you check your CSS and your HTML around the graphic, in the ePub in question.
The attached epub book contains three gif files. Gif file si64.gif is not rendered in my ADE. The paragraph is repeated twice, once with the div elements around the gif files, and once without any div element. In both paragraphs the problem is present.
The screenshot shows a rendering of the xhtml file in firefox, with si64.gif.