I’ve put an image on the track and used Blur:Box for some effect over the duration of the clip.
It somehow also made a blurry frame around the image which doesn’t look good, looks like a blurry grey border. The background of the image is white so it’s quite noticeable.
A picture paints a thousand words. If you post an image of the problem you would make it easier for someone to understand the problem better and be able to help you sort it out.
As a supplement of information about the Blur: Box filter:
If you add a Alpha Channel Adjust filter (mode: Blur) under Blur: Box, you’ll still get blurry edges, but they will look much better.
(Right-click on the GIF and open it in another window or tab to se it at 100%)
Sometimes you can also add the Unpremultiply Alpha filter under Alpha Channel Adjust, for a different blur effect on the edges. It removes the dark areas in the blurred edges.
(Right-click on the GIF and open it in another window or tab to se it at 100%)
These filters, Alpha Channel Adjust and Unpremultiply Alpha can also be used with with Blur: Gaussian, Blur: Exponential and Blur: Low Pass when you need to add a blurry edge.
Here’s an example with Gaussian Blur:
(Right-click on the GIF and open it in another window or tab to se it at 100%)
I got carried away a little bit and tried to apply a motion blur to a flying rocket using the files belllllow, 5 tracks and a few filters, including Blur: Box:
Isn’t the meaning of bellow is a deep roaring shout or sound, however, I think you got a typo in the haste of typing a post. Next time remember to remove that extra él or L, perhaps it’s a modern style to write below that I didn’t know till now.