Unless you are using a custom theme with special functionality, the text that appears when you hover over a product image can be changed by editing the product >> Pictures >> and changing the value appearing in the "Alt" column. If you want nothing to appear, leave it blank.
On the catalog pages (category and manufacturer and search results) the text that appears when you hover over the images is a derived value and can only be affected by editing the code view.
/Views/Catalog/CategoryTemplate.ProductsInGridOrLines.cshtml - approx line 80
Look for this:
<div class="picture">
<a href="@Url.RouteUrl("Category", new { SeName = item.SeName })" title="@item.PictureModel.Title">
<img alt="@item.PictureModel.AlternateText" src="@item.PictureModel.ImageUrl" title="@item.PictureModel.Title" /></a>
</div>
To remove the text that appears on hover of the images, change it to this:
<div class="picture">
<a href="@Url.RouteUrl("Category", new { SeName = item.SeName })" title="@item.PictureModel.Title">
<img alt="" src="@item.PictureModel.ImageUrl" title="@item.PictureModel.Title" /></a>
</div>
Same with the ManufacturerTemplate.ProductsInGridOrLines.cshtml and Search.cshtml in the same folder.