It's not DPI you worry about when posting on the Web, worry about dpi for making Prints. The standard 72 dpi is fine for posting on the Web, what you need to worry about is Resample/Resizing and Jpeg Compression. Depending on how large the original is and what Image Edititing software you use will dictate what procedure you want to use. Getting maximum size and quality to fit the Upload limits is a Tradeoff you need to experiment with to find one that suits you. Rendo Dimension limits are pretty liberal at 4,000x4,000 it's the meager 512k footprint that's a problem.

Most decent Image Editors give you good control of Resizing/Resample (some have a wide range of Resample algorythms to choose from and some don't) and also a control of the ammount of Compression you use. I have several different ones I use that have good Sizing and compression control, PhotoFiltre(Free or StudioX versions), Paint.net, Project DogWaffle, Gimp, my favorite by far though is PhotoShop.

I use PhotoShop for my Resizing/Resampling and the Save for Web option for the Images I post on the Web. I have found that resizing from my standard 4,000x2,000 PNG format originals to 1800x900 then I can keep the Jpeg compression between 70-80% on saving for web which will keep reasonably good detail and allow me to keep the image just under the 512K limit here. It's not a one set of numbers fits all images thing, image content(complexity), Gradient etc, will determine the amount of Resizing and Compression needed to reach the Limits you need.

Remember...No matter where you go there you are