Please remember that students often use mobile devices to access your learning materials.
Uploading large PowerPoint presentations may cause issues for your students when trying to review materials. Inserting a background image not only adds size to your slides, they might also not be transferred across all devices or through all browsers. So, for example, white text would then be displayed on a white background, looking as though the slides are blank.
It is good practice to add a coloured background, to aid dyslexic students. Please consider changing the background colour of the slide master from white rather than adding a graphic. Or, where you have already created multiple slides with graphics, it is helpful to also upload a .pdf copy of your presentations.