Canva is an online graphics creation application which also has an associated iPad editor that links to your free account. There is a fair range of free images included but only simple backgrounds, but you can easily upload your own images. A number of useful templates are included (including infographics) which does deal with common sizes for different online applications. The ‘Blog image’, for example, could be used as a NILE banner image.
Images can be exported as PDF (text included, so suitable for Turnitin submissions) or a PNG image.
It is possible to share editing between a number of users but this workflow has not been tested. Though it does claim to be simple to use it is fairly demanding on resources and may not be suitable for an old PC/browser or many mobile device browsers. If you are comfortable with PowerPoint and sizing images created there, this may be overkill. But it could equally be a tool that you enjoy and can use regularly.