You should really ask the printing company for advice. A newsletter, so probably offset printing. Photographs will be 'half-toned' ie. made up of little dots. These tend to look fuzzy so the advice is over-sharpen before submitting. Nothing going to compensate for a tiny image scaled up and pixellated.
For the colour edition even more important to find the printers requirements. Often images are needed in CMYK colour-space and Gimp is strictly RGB. Still do your editing in Gimp and use free conversions such as
https://www.rgb2cmyk.org/ Might not be needed if the printer is customer friendly and does the conversion for you.