All images that are longer than 1920 pixels in width or height will be resized to 1920 pixels. To avoid quality loss make sure your images are 1920 pixels or smaller, in width or height.
Dpi (dots per inch) only matters when it comes to printing or scanning. Monitors don't have dots but pixels, so dpi value has value has no effect on the quality of an image viewed on a screen or uploaded online.
Image color profile
Before uploading your images, make sure their color profile is sRGB. If you are using Photoshop go to File > Save for Web. This will make sure that the images will be optimized for web browsers.
The file size of an image is measured in kilobytes (K), megabytes (MB), or gigabytes (GB). The file size is affected by the pixel dimensions of the image and its format. Because of the varying compression methods used by GIF, JPEG and PNG formats, file sizes can vary considerably for the same pixel dimensions. The image file size cannot exceed 12Mb to be uploaded on Portfoliobox.