I would check out the following document...it shows how to construct a multi-part request with an image to the V3 image. This should work on Jive 6 and 7.
This should help get you squared away.
Hope this helps.
Thanks for the reply Ryan,
I eventually figured this out using some examples of yours from other posts, I was able to hobble together a pretty groovy group image creator
in case anyone cares, I did it like so...
-output from canvas in base64 (I used jcanvas jquery plugin which made the canvas stuff easy)
-convert this to a blob, I used jquery-image-blob plugin
-append this blob to formData
-make image using /v3/images endpoint with multipart/form-data
-set the place avatar using /v3/places/[placeID]/avatar?uri=[url for image u made]