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Yola uses image optimisation in some parts of the system. A file of that size will slow down loading times remarkably and will possibly penalise your site for the slow load time. Were you using the image widget? If so you should be able to turn off image opt. I would manually reduce the image file size to approx 100 Kb for a high res image for web-pages.
You say that your image was reduced to 750Kb. That is still a huge file and wouldn't be the cause of a reduction in quality of the image on the screen. There's something else happening here. The image opt facility is pretty poor in quality and for that reason it;'s best to do it off-line and turn off this feature for the image.
For a good rendering of an image something in the order of 100 Kb is generally adequate for web-pages.
You may also want to check out our tutorial on optimizing your images. Also, you can turn off image optimization should you wish, but as Gop correctly pointed out, 1.5MB is very large and is likely to affect the speed of your website loading for your visitors as well as for you in your Sitebuilder.