What file format is the image you uploaded - i.e. what did you export it from AI as?
There’s this known issue (which I assume is still an issue), but if you’re certain that what you uploaded does have transparency (is either a PNG with alpha channel or a proper SVG), then the transparency should be preserved.
EDIT: You can check whether the “master” image in your Bubble storage has transparency by going to Data → File Manager → Search and then clicking the “view” link for that image to see the original file you uploaded. Images appearing in Bubble pages are automatically resampled / optimized, and that’s generally a very good thing. However, it means that what you see on a page is usually not the actual file you uploaded but rather an auto-optimized derivative.