Digital Banner Ad designer, HTML5

We are looking for a freelance designer to help us build mockups & banner ads for our digital advertising clients. For the mockup part we mostly are looking for static banners, a jpg or gif created in any creative programme of choice (eg. Photoshop) would be sufficient - this would be for quick mockup creation where we want to showcase our ideas to the client. Time is key here - we usually have less than 48 hours for requests like this, and most of the time there won't be much guidance or any assets from the client. This means the designer has a lot of flexibility and can be rather creative with their ideas. Sometimes we will require animated banners to show some fancy transitions etc - again if this is for a mockup, the format doesn't matter that much, so a gif would be fine. In some instances, we need to build out the actual creatives in HTML5. This will mostly contain animation, and ideally would be done either straight in our Ad server Sizmek (which offers an ad-builder programme), or done separately according to the Sizmek specs for a seamless upload. For the actual build of the HMTL5s, we don't have such a tight turnaround time, and will get assets & brand guidelines etc for the designer - here quality is key and we want to make sure to impress our clients. There's also a lot of attention to detail needed - we have worked with designers in the past that put in spelling mistakes, used wrong font sizes etc - this is something we definitely want to avoid.