You must be using the hourglass chart. I get the same pixelation on .emf, .bmp, .png and .jpg for this report and not on box chart. It appears as if RM is using thumbnails rather than resizing the high rez picture. Its a Bug!
When you open file>export there is a drop down by the file type that you use to select the format, .emf, .bmp, .jpg, etc. I have no problems using this to select .jpg or .png to save.
RM does not save any files as read only unless their properties by definition makes the uneditable, eg .pdf. Photo editing programs read the file properties on open and if they are not set as read only before open, they are editable. To make the uneditable you must change them to read only outside of RM (Windows File Explorer can do this).
Charts has been a weak part of RM since its introduction and the hourglass report was released as an update to RM7. They have stated that it was to be rewritten as a part of RM8.
All my testing was done with Irfanview, a free photo editor that has more capabilities than Paint, (more like Photoshop) and verified with Paint.