In practical terms, the big difference between jpeg and tiff is that jpeg uses what's known as "lossy" compression to reduce file size. Lossy compression simply means that image data is discarded to accomplish this reduction. Areas of similar colour, for example, will be sampled and all similar image data then "lost". When opening the file the image data sampled will fill in all the similar areas. You may have noticed that jpegs featuring large areas of blue sky have smaller file sizes than images of equal pixel resolution that have greater detail.
That's why when submitting to Dreamstime you have to use the highest quality setting when saving to jpeg so that the least amount of image data is discarded.
No image, once downloaded, should remain in jpeg format. I say this because each time you save a jpeg, as a jpeg, more image data is discarded and therefore more detail and quality is lost. Tiff uses a lossless compression scheme and so even though a compressed tiff may not be as small a file as a jpeg, it will not lose detail no matter how many times you save it.
Macro USM, Sigma OS 70-200mm f/2.8 EX DG HSM, Sigma 2X Tele-converter...