Can I create custom upload form in SharePoint for a picture library so that I can add code behind methods to it and do some filtering? I already tried event receivers but no gain only pain for multiple items, as if their dimensions were too long. Any type of tutorial will be fair enough.
Yes, you can create a custom upload interface. I recommend either via a ASP.Net application page or a web part. You will be able to control the entire upload experience using your own interface like these. HTH