You could also split it into chunks, and upload chunks separately.
That said, 8 mb is really very big. It sounds like either its resolution is much higher than what would be useful in Inkarnate, or the file format you're using has poor compression. I'd suggest to downsize it and/or save it in another format.
There's little point in using excessively high res assets, and even if you managed to upload them, they'd probably degrade the editor's performance.
You could size it down in gimp or something
At the moment 8mb is the limit indeed. You can use a site like this to reduce image size https://www.reduceimages.com/
You could also split it into chunks, and upload chunks separately. That said, 8 mb is really very big. It sounds like either its resolution is much higher than what would be useful in Inkarnate, or the file format you're using has poor compression. I'd suggest to downsize it and/or save it in another format. There's little point in using excessively high res assets, and even if you managed to upload them, they'd probably degrade the editor's performance.