Optimising images for web

[emerald]

Hello, I have problems optimising my images for the web. I create large size PS compos 50x25cm at 150dpi with Photoshop and Illustrator. (Any larger and I would have problems shifting them on my Imac). I combine them with typography from either PS or AI. When I view them in Preview they look sharp, when they appear in websites full screen they look pixelated, especially the typography which as said in some cases I combine with those images. I have read some previous threads on this, and have tried numerous variations of file types- and sizes but so far to no avail. The images still look better than the type by a small degree. I did a 'save for the web'. Tried .png, tried .jpg, and .gif but not a lot of difference. I tried really big files with 150dpi, brought down to 72dpi, before saving for the web. I tried outlining type in AI before importing over the pasteboard into PS; no difference... It slowly drives me potty... Would it be the difference in me using Safari or Chrome? Would it be the server of a particular site? Do I need to go into something like Quark or Indesign to combine the art and save it from there for a better result...?