I’m not sure if this is the cause of the error code, but was wondering why you are using an svg wrapper to insert an inline image (gif)? I would use Sigil’s built-in "Insert image" rather than the "InsertImageSVG" plugin.
I only use the wrapper if I want an image to take up the whole page/screen and maintain the aspect ratio (xMidYMid meet).
If I want to insert the image amidst paragraphs then I would wrap the <img> tag with a <div>:
Code:
<p>Some paragraph text.</p>
<div class="image">
<img alt="" src="../Images/qfig07.gif"
</div>
<p>Some paragraph text.</p>
And if I want to embed the image within the paragraph (last resort):
Code:
<p>some text with one-off symbol <img alt="" src="../Images/weirdsymbol.gif"</p>
Of course, there are also the other ePub3-isms, like using <figure> instead of <div> if you are making an ePub3 book…