Can I upload Image Sequences?

Not yet! But here some workarounds...

A quick Web search of "convert image sequence to video" will yield a variety of ways to convert image sequences to video formats supported by SyncSketch using commercial tools you may already have, or with freeware utilities.  Look for tools that support automated workflows that integrate into your toolchain, whether it's a GUI app that watches your output folder, or a command-line tool like ffmpeg that can be easily scripted in Python or a similar language.

If you are on an Enterprise plan, you can easily add the SyncSketch Python API to a Python script and get it to upload the converted movie directly to a SyncSketch review automatically.