Docs: suggest reducing image size for new quests (#5937)

* general advice about reducing file size manually

* mention online tools to simplify size/quality comparison
from discussion at Helium314#644 (comment)

* emphasize playing with quality, without recommending exact values

* mention online SVGOMG

* mention GIMP

* do not mention values at all

CONTRIBUTING_A_NEW_QUEST.md

@@ -336,7 +336,7 @@ Keep similar style to existing ones and the app in general. Note that the backgr
 
 Once the quest icon is ready:
 
-- when using Inkscape, save as "Optimized SVG" to remove unnecessary cruft or use another tool for that, like [svgo](https://github.com/svg/svgo)
+- when using Inkscape, save as "Optimized SVG" to remove unnecessary cruft or use another tool for that, like [svgo](https://github.com/svg/svgo) or online [SVGOMG](https://svgomg.net/)
 - Put SVG into [`res/graphics/quest`](res/graphics/quest) folder
   - SVG is a standard format editable in various software, unlike internal Android Studio XML that will be produced in the next steps.
 - Open Android Studio
@@ -464,6 +464,10 @@ Each of these folders should hold the same image resized to a different resoluti
 
 The [rescaling script](https://github.com/matkoniecz/rescaling_for_android) may be useful, but you can also do this manually with Gimp or similar software.
 
+Please make sure that the images do not take too much disk space. Most useful way to do that is by lowering JPEG quality, which can make images noticeably smaller. Play with settings to see which is the smallest size which does not degrade image quality visibly.
+
+[GIMP](https://gimp.org/) allows such previews while saving JPG files, and there are also online tools like [squoosh](https://squoosh.app/) which allow for quick visual comparison if you prefer that.
+
 After adding a photo, remember to update [the credits file](app/src/main/res/authors.txt) (different to the one for icons).
 
 ## Resurvey