An upload is in fact not a single upload. Roll20 uploads each image multiple times with different granularities, in the sequence: thumb, mini, med, max and original. When you X away the progress bar window, uploads continue in the background (F5 or closure of the browser tab stops the upload). Uploads can take a long time when uploading large, detailed images (reason: jpg's are still converted to png's and a 10Mb jpg can become a 50Mb png). You can monitor the progress of the upload in the network tab of the developer bar of your browser, below you can see the last two http requests the browser makes for a successful upload. Notice the 46.48 MB upload size of the 8 MB jpeg. The sure way to verify whether an upload really finished is to select the image on the VTT, press Z and inspect the image. If it is correct, the upload succeeded.