Hi again @Pablo_Sektman!
I'm really sorry about the trouble here. Because we changed the way projects (and their assets) are stored, it can be confusing going back and forth between myBalsamiq and Desktop.
It is totally possible, though, and may only take a small tweak to your workflow.
If you export the project as a BMML Zip (https://docs.balsamiq.com/desktop/exporting/#exporting-for-use-in-a-previous-version), and then just upload that zip to your myBalsamiq site, that guarantees that all the assets will make it with your mockups.
Otherwise, you will have to upload the mockups and assets separately (as you have done).
I see the missing image in that mockup. Was it one of the ones you uploaded? If you upload the assets, and then the mockups, they should be able to reference the images properly.
If you want, shoot me an email at support@balsamiq.com and we will dig into this 