Hi and welcome to the forums!
The import will always upload the original image to your assets library, which remains accessible to you in all its original resolution’s glory. However, the images are optimized for Foundry use which includes downscaling them for high resolution maps, in order to avoid potential issues for some players.
You can read more about The Forge’s image optimizer (and check the ‘Read More’ button too for details on the possible consequences of disabling the optimizer) here : The Image Optimizer
If you browse for the file in the scene configuration, you should see a “Disable optimization” checkbox at the bottom which would allow you to get the full high resolution image, but make sure you read the link about and understand what it means.
I hope that helps!