By default, PhotoRoom automatically exports images in the best format that suits your need: .jpeg to keep size small, or .png to respect transparency.
If you need to always export in .jpg, regardless of the transparency, you can select it. In PhotoRoom, go to "Your Content" tab and then "Preferences" to enable "export in .jpg". It will export images that are smaller in storage space but can sometimes be of lower quality. The setting is saved and will be applied to all your future exported images.
Please note that this setting is currently not shared through your team(s) and not shared through your platforms. You need to set it individually per team member and once per platform (web, iOS, Android).