I opened a support ticket with LinkedIn who almost immediately closed said ticket and instead directed me to ask a question here.

Apologies for the question but it appears the form to create a new app requires an image upload for the app logo, but jpg, jpeg, png all fail with a message saying the image should be square at at least one dimension is 100px.

The image I resorted to testing with is a 100 x 100 black square, as everything else I tried also failed I suspect this is an issue with the validation of the image and not my form request.