While a purchased download is always a high resolution image/video, and image downloads will always have a resolution of 300 dpi, this doesn't enhance the quality of a low resolution image uploaded into your design.
A low quality image can result in a pixelated poster, flyer or web graphic. This issue is more acute in larger designs such as a banner. Sometimes, image enhancement softwares online, such as letsenhance.io can help improve image quality but this doesn't work in all cases.
More often, the image will need to be replaced with a larger image. Here's what you need to do:
Fixing your order
To make changes to your download order, go to 'Order History', click 'Make a Correction', and you will be redirected to the editor. Replace the low quality images with higher quality, crisper images (stock images always guarantee high quality). Once you've finalized changes, click 'Save and Recreate Order'. Your order will now regenerate, which typically takes 2-4 minutes. Get the download via Order History, or your email.