Hello Bushturkey -
In your Account Preferences there is a section for Image Management - and we currently offer 2 sizes for the maximum image size for your image uploads. The higher of the two is 1920 x 1440px, so uploading larger images will result in some mild compression to bring your images to that size. Unfortunately, at this time, we have no larger option to select. Typically compressing an image would not have an adverse affect of the image quality as it might when an image is enlarged.
You should be able to use your larger images for upload and the reduction in size should not have an adverse affect on the actual image quality. Alternately, if you have image hosting elsewhere online, you can shift the description editor to use externally hosted images; this can be done by clicking the linkg for "switch to externally hosted images" in the Image Selection area of the lister page, which is just above the section for Optional eBay Gallery Images.
You can also make a feature request regarding this at our Feature Center located here.