first, make sure your sequence is named something like myfile.0001.tga. It is very finicky about the sequnce name. (i think you can also use myfile.0000)
Second, by default when you turn on the image sequence feature, it corresponds the frame # to the image sequence #. (so frame 23 plays back frame 23 of your animation). You can break the connections on this by right clicking and then you can keyframe it manually, or write your own image sequence expression.