You can do it, but it's a bit convoluted ..
1) Enter your text and upload your gallery photos ... (save draft)
2) Go to the Media tab .. click "Add New" (you'll find it easier if you open in a new window or tab)
3) Upload the large pic image (NOT CONNECTED to your post)
4) Return to your post, place the insertion point on where you want the big pic. Click the "Upload/Insert" icon (I'm assuming you are using WP 3.3, if not the photo upload icon)
5) ... in the resulting window, click the "Media Library" tab and select the large pic with whatever options you need (alignment, links, etc) ..
... insert .. and save/publish ..
... and there ya go.
Note .. you may need to rework your template design a bit if you see the gallery wrapping around the large pic .. but that's a whole 'nother story.
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