Change the file (jpeg, gif or swf) according to screen size?

You can use images as divs and use media-query to set an example if you have two images:

<style>
.banner-1 {
    background: url(images/normal/banner1.jpg);
    display: inline-block;
    width: 978px;
    height: 90px;
}

.banner-2 {
    background: url(images/normal/banner2.jpg);
    display: inline-block;
    width: 500px;
    height: 50px;
}

@media (max-width: 420px) {
   .banner-1 {
        background-image: url(images/celular/banner1.jpg);
        width: 230px;
   }
   .banner-2 {
        background-image: url(images/celular/banner2.jpg);
        width: 100px;
        height: 10px;
   }
}
</style>

<div class="banner-1"></div>
<div class="banner-2"></div>

Set @media (max-width: 420px) to what you think is appropriate for the cell phone screen, since swf would not be able to change, but usually the swf itself can be expandable, so just add width: 100%; to adjust to the DIV element which receives width: and height of the media-query for example, however I should warn you that Google Chrome and Firefox already block by default swf files (I believe you can activate or still have the click to play , but it's something that will really only work depending on the user, flash is no longer a good way for some time)

See if it catches, it will detect if it is mobile and call a file and otherwise call another file

function isMobile(){
    var a = navigator.userAgent||navigator.vendor||window.opera;
    if(/android|avantgo|blackberry|blazer|compal|elaine|fennec|hiptop|ip(hone|od|ad)|iris|kindle|lge |maemo|midp|mmp|mobile|o2|opera m(ob|in)i|palm( os)?|p(ixi|re)\/|plucker|pocket|psp|smartphone|symbian|treo|up\.(browser|link)|vodafone|wap|windows ce; (iemobile|ppc)|xiino/i.test(a)||/1207|6310|6590|3gso|4thp|50[1-6]i|770s|802s|a wa|abac|ac(er|oo|s\-)|ai(ko|rn)|al(av|ca|co)|amoi|an(ex|ny|yw)|aptu|ar(ch|go)|as(te|us)|attw|au(di|\-m|r |s )|avan|be(ck|ll|nq)|bi(lb|rd)|bl(ac|az)|br(e|v)w|bumb|bw\-(n|u)|c55\/|capi|ccwa|cdm\-|cell|chtm|cldc|cmd\-|co(mp|nd)|craw|da(it|ll|ng)|dbte|dc\-s|devi|dica|dmob|do(c|p)o|ds(12|\-d)|el(49|ai)|em(l2|ul)|er(ic|k0)|esl8|ez([4-7]0|os|wa|ze)|fetc|fly(\-|_)|g1 u|g560|gene|gf\-5|g\-mo|go(\.w|od)|gr(ad|un)|haie|hcit|hd\-(m|p|t)|hei\-|hi(pt|ta)|hp( i|ip)|hs\-c|ht(c(\-| |_|a|g|p|s|t)|tp)|hu(aw|tc)|i\-(20|go|ma)|i230|iac( |\-|\/)|ibro|idea|ig01|ikom|im1k|inno|ipaq|iris|ja(t|v)a|jbro|jemu|jigs|kddi|keji|kgt( |\/)|klon|kpt |kwc\-|kyo(c|k)|le(no|xi)|lg( g|\/(k|l|u)|50|54|e\-|e\/|\-[a-w])|libw|lynx|m1\-w|m3ga|m50\/|ma(te|ui|xo)|mc(01|21|ca)|m\-cr|me(di|rc|ri)|mi(o8|oa|ts)|mmef|mo(01|02|bi|de|do|t(\-| |o|v)|zz)|mt(50|p1|v )|mwbp|mywa|n10[0-2]|n20[2-3]|n30(0|2)|n50(0|2|5)|n7(0(0|1)|10)|ne((c|m)\-|on|tf|wf|wg|wt)|nok(6|i)|nzph|o2im|op(ti|wv)|oran|owg1|p800|pan(a|d|t)|pdxg|pg(13|\-([1-8]|c))|phil|pire|pl(ay|uc)|pn\-2|po(ck|rt|se)|prox|psio|pt\-g|qa\-a|qc(07|12|21|32|60|\-[2-7]|i\-)|qtek|r380|r600|raks|rim9|ro(ve|zo)|s55\/|sa(ge|ma|mm|ms|ny|va)|sc(01|h\-|oo|p\-)|sdk\/|se(c(\-|0|1)|47|mc|nd|ri)|sgh\-|shar|sie(\-|m)|sk\-0|sl(45|id)|sm(al|ar|b3|it|t5)|so(ft|ny)|sp(01|h\-|v\-|v )|sy(01|mb)|t2(18|50)|t6(00|10|18)|ta(gt|lk)|tcl\-|tdg\-|tel(i|m)|tim\-|t\-mo|to(pl|sh)|ts(70|m\-|m3|m5)|tx\-9|up(\.b|g1|si)|utst|v400|v750|veri|vi(rg|te)|vk(40|5[0-3]|\-v)|vm40|voda|vulc|vx(52|53|60|61|70|80|81|83|85|98)|w3c(\-| )|webc|whit|wi(g |nc|nw)|wmlb|wonu|x700|xda(\-|2|g)|yas\-|your|zeto|zte\-/i.test(a))
        return true;    
    else
        return false;
}

if(isMobile()){
    document.write('<link rel="stylesheet" type="text/css" href="mobile.css" />');
}else{
    document.write('<link rel="stylesheet" type="text/css" href="desktop.css" />');
}

. No document.write can put whatever you want, an image, css file call, etc ... or even an entire HTML.

document.write('<img src="http://site/aaa.jpg">');