If you’re creating image (‘banner’) ads, it’s time to pension off the old JPEG or GIF formats. The quality is no longer good enough in comparison with the text – and most images – surrounding them, and will nearly always make your company look second-rate. Your designer should be using ‘HTML5’ now; if you’ve seen really high quality ads around the web, that’s almost inevitably the format in which they were created.
Using HTML5 is a new skill for designers, but any worth their salt will already have this under their belt. If you do your own design work, there are a number of HTML5 editors around, and it’s a good idea to learn how to use one. Google Web Designer is worth experimenting with.
I’ve questioned direct-booking ads with websites in the past. The vast majority of sites aren’t sophisticated enough to offer anything other than fixed advertising rates with no guaranteed response, and in 2018 that’s just not good enough (it wasn’t really good enough in 2008, but what can I say?). However, it may be the only way to get on a site that you’re either forced to use, or which has proven response even at fixed rates. If so, and you have to supply ads to fit their formats, HTML5 is the way to go.