Unsplash provides free high-quality photographs–for any type of use. You can edit the photos however you want and add them to your own blog graphics. They add new photos every ten days, so it’s likely you’ll find what you’re looking for.
Pixlr Express will give you quick access to a plethora of photo editing tools. You can crop, rotate, and resize, but also add text, stickers, or overlays. The site isn’t perfect, but when combined with other free tools, it can be pretty powerful. They also frequently add limited edition overlays and effects, which is fun to check out.
PicMonkey has become a staple website for many bloggers. Use it to make collages or edit photos. You can also add text, effects, and stickers. However, you can’t save work, so if you want to make sure your graphics are consistent, take detailed notes on what font colors, sizes, etc. you used for your graphics.
UPDATE: PICMONKEY NOW CHARGES A MINIMUM OF $3.99/MONTH IF YOU ACTUALLY WANT TO BE ABLE TO SAVE/DOWNLOAD YOUR IMAGES. YOU CAN CREATE/EDIT IMAGES FOR FREE, BUT YOU’RE ONLY GOING TO GET A COPY BY SCREENSHOTTING YOUR CREATION; YOU CANNOT SAVE IT TO YOUR COMPUTER FROM PICMONKEY ITSELF WITHOUT PAYING.
Canva is a graphic design site. Use it to make headers, social media images, blog title graphics, infographics and more. There’s some free clipart on the site, and you can choose to pay for access to more. However, the site often works best when you upload your own photos. However, it doesn’t really have photo editing tools, so make any desired changes on Pixlr or PicMonkey first. One of the best features of the site, however, is that it saves designs, so if you realize you made a graphic with a typo, you can just log back in and edit it.
ColorZilla is a free Chrome/Firefox extension that will allow you to use an eyedropper to copy the HTML hex codes of colors you find on the web. Use it to make sure your graphics all match your theme, or use it pick up color from book covers. It will also analyze the color scheme of a webpage for you or help you generate CSS gradients.