Hi sboerner!
I’ve been working along the same lines - swapping out 150+ images in an animation experiment, You can read more about it here.
I bring the image itself into Hype’s animation pane (no placeholder Rect, etc.). Below is an example tag I place in the innerHTML.
<img src="${resourcesFolderName}/smoke_1000.png" width="600" height="400" id="imageSeqHolder">
Note the “id” assigned in the tag.
Then similar to what You wrote:
$("#imageSeqHolder").attr("src","${resourcesFolderName}/"+"smoke_"+imageNum+".png");
I’m using: “+“smoke_”+imageNum+”.png" - in this case because I’m running an animation and that bit of code allows me to run the sequence, so “smoke_1000.png” is then replaced by “smoke_1001.png” etc. in the image tag. Your code should work fine.