Big problem with the watermark

[Vigla]

Dear friends,

    I am using Videopad. I put my watermark to one video and it was fine. The next time that I opened the video it was fine. But the next-next time that I opened the video, the watermark had big problem. Ιt is totally corrupted!! Some red letters that it has, look like red flowers! The graphic design has spoiled and is blear. I reintroduce it, it looks good when it's big but a little blear and when I make it small it's totally destroyed. Why is this happening and how can it be repaired?
    I am thinking that maybe there is a problem with the preview of the video but there wasn't at the beginning. If that's the problem how can I fix it??

Thank you very much!