To make your existing pictures show up full size on your blog, you'll need to edit all your existing posts. Click Edit Posts and select which post to edit first.(Picture 6)

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Now, you'll have to go into "edit html" mode and edit the codes of all your uploaded pictures.
Below is a picture of what you need to look for. (Picture 7)

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I circled two pieces of code with red. The first one is the altered code, the second one the original.
You need to change the S400 into S800 in the HTML code of all your pictures. That's quite a bit of work, but that will do the trick - in most cases.
Our next picture is a screen shot of the preview, so you can see the difference!(Picture 8)

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You can, of course change all these codes manually, but if you've got MS Word installed on your computer, there's a much quicker and easier way to do this.
When you go into html view, copy and paste everything into Word. Place the cursor somewhere in the post field and press the "Ctrl" and "A" button, thus selecting everything within this field. (Picture 9)

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Press "Ctrl" and "C" to copy the contents of your post, and paste it into a new Word Document using "Ctrl" and "V".Then hit Crtl + F and select the tab "replace"In the "Find what" field, type in s400and in the Replace with field: s800Then click replace all. (Picture 10)

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After that, copy and paste your edited Word document into the HTML field of your blog post and hit publish.
Now, some of your pictures might actually be too big to fit your blog, but don't worry. All you need to do to get those pictures fit into your blog nicely again, is copy and paste the following piece of code into the HTML-code of those pictures:width="600" height="450"as shown in picture 11.

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The value that's most important here, is the
width="600". You will need to calculate the height of your picture accordingly (or let your picture editing program do that for you and just copy and paste the values it gives you) or simply leave out the values for height. Either way works.