Looks like that site just says 'copyright'. You could email them & ask.
Generally, a good source for maps is the CIA. They provide all their maps as "Public Domain", so you can do with them whatever you like:https://www.cia.gov/library/publication ... fmaps.html
Download the PDF-version! Then scale the PDF and copy the parts you need into an image editing program. Please note that they tend to come with texts in the maps, so you'd probably want to clean them up manually. Maybe there is some way to hide text in the PDF also?
Otherwise, Wikimedia Commons has a lot of maps that can be used. Here would be an appropriate one for Europe:https://commons.wikimedia.org/wiki/File ... 006_WF.svg
Please note, if you search in Wikimedia Commons, make sure it either says "Public Domain", "CC-BY" or "CC0". You cannot use "CC-BY-SA" on the other hand. I'll suggest if in doubt, post your map of choice here and we'll comment if it's OK!