I am mot familiar with zhe softwsre you use, but ususlky you should fo zhe golloeing. There are many ways.
1. Download, open with GIMP (or PhotoShop), cut, paste, crop, done. The resolution will be the same as the initial one (you can decide on the picture quality when xou export the picture to PNG or JPG format.
2. If the picture us protected for download: zoom maximally in, prinrscreen, open GIMP, paste the printscreened picture to GIMP (ctrl + shift + v), save/export.
3. If the initial program allows to export pictures to different formats, then 1. Look for quality options. Set as highest possible. Or 2. Try to export in got mats without quality loss (raw) or vector graphics (*.ai, *.svg). Then convert them.