I don't believe Gif's are supported, maybe someone else can verify that.
What you can do is duplicate your prop, make sure Unique is selected in the modify section
Then set each prop to a different texture file. Then on the timeline, change the Modify - Display from visible to invisible for the original one, and change the second prop to be visible at the same keyframe.
It's not as easy as a Gif, but it works.
If you want it to change a lot of times, I wrote a Python Script for IClone 7 that does some similar. I might be able to convert it to IClone 8 if you need it.
Maybe someone else can tell you for sure if Gif's are supported or not.
Creator of the free website:
PixelDatabase.Net
A Free Online Text Based Image Editor