There's 4 positioning you can use:
1. Static is the default position and it will make the element appear exactly where it exists in the code. You can't use that.
2. Relative positioning is much the same expect that it creates a reference point for absolute positioning and you can displace the relatively positioned element by adding the left/right/top/bottom properties. Displaced element will appear elsewhere in the page but still occupy its existing space.
3. Absolute positioned elements are taken out of the flow of the document and appear on top of the document, similar to a post-it note appears on a book page or printed text. Absolutely positioned elements do not interact with other elements on the page (they don't push the content around, they simply float over [or under] the content). Absolutely positioned elements are positioned according to the first positioned (relatively or absolutely) parent. If such parent does not exist, they are positioned according to the viewport.
4. Elements with fixed positioning are taken out of the flow (and behave much like the absolutely positioned ones), except they are always positioned only according to the viewport. This further means that when the viewport is scrolled, the elements do not scroll off the page, but remain in view -- because they are not fixed onto the webpage (which is being scrolled) but on the browser window (which is always pinned down). A similar thing to fixed positioning would be a post-it note on a glass cover of a book. You can flip through pages, but the post-it will still remain visible.
Given all these explanations, you need to go with absolute positioning. Furthermore, you need:
1. To make sure the first parent of the absolutely positioned element (span) will be the appropriate element which will allow precise positioning of the photo over the previous one. The best one seems to be #mainContent, although #container could work as well. This will require you to remove relative positioning from the .gallerycontainer (and any other parent on the way to mainContainer).
2. Adjust left/right/bottom/top properties to match the ones of the photo on the right. Remember that you're not limited to positioning from the top-left corner of the element -- you can do it in the top-right, which seems more appropriate for you. In addition to that, note that you can use negative values to place the element outside the parent container.
This should allow you to precision place the image where you want it -- and the image will stay there regardless of the window size. Also note that given the proper use of absolute positioning, there should be no additional gaps at the bottom.
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